media: v4l: fwnode: Update V4L2 fwnode endpoint parsing documentation

The semantics of v4l2_fwnode_endpoint_parse() and
v4l2_fwnode_endpoint_alloc_parse() have changed slightly: they now take
the bus type from the user as well as a default configuration for the bus
that shall reflect the DT binding defaults. Document this.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>